In recent years, a movement has gained traction to replace traditional incandescent and fluorescent lighting fixtures with lighting fixtures that employ light emitting diodes (LEDs). Compared with traditional lighting fixtures, LED-based light fixtures are much more efficient at converting electrical energy into light, are potentially much longer lasting, and are also capable of producing light that is both very natural and excellent at rendering colors. As a result, lighting fixtures that employ LED technologies are rapidly replacing incandescent and fluorescent fixtures in residential, commercial, and industrial applications.
Unlike incandescent and fluorescent lighting fixtures, an LED-based lighting fixture requires relatively complex electronics to drive one or more LEDs, and in many instances, multiple strings of LEDs. The electronics generally include power conversion and supply circuitry in addition to special control circuitry to provide uniquely configured signals that are required to drive the one or more LEDs in a desired fashion. While the LEDs themselves are solid-state devices that tend to operate over greatly fluctuating temperatures for long periods of time, the electronics associated with the LEDs are often prone to failure. These failures generally stem from component failures as well as printed circuit board (PCB) failures that are caused by the repeated cycling between relatively low and high temperatures.
Solder joints are a source of the PCB failures. Solder joints are used to electrically and mechanically couple the leads of electronic components to the vias of the PCBs. The PCBs are often mounted in a housing, and the open regions between the PCBs and the housing are filled with a potting material. The potting material helps to dissipate heat generated by the electronic components, as well as protect the electronic components from problematic environmental elements, such as water and conductive debris.
Silicone mixed with thermally conductive fillers has been widely adopted as a potting material for LED-based lighting fixture applications. Since silicone remains relatively soft, silicone has traditionally been considered a low-stress potting material. Unfortunately, while silicone is relatively soft, its relatively high coefficient of thermal expansion (CTE) causes an inordinate number of solder joint failures. Since the potting material is in contact with and generally surrounds the electronic components, the expansion and contraction of the silicone results in significant and repetitive forces being imparted on the electronic components that are mounted to the PCBs. These forces are transferred down the leads to the solder joints such that radial forces, axial forces, or a combination of radial and axial forces are applied to the solder joints. Over time and after numerous temperature cycles, these forces cause fractures in the solder joints. The fractures in the solder joints cause breaks in signal paths, and thus, premature failures of the circuitry provided by the lighting fixture. Even if the initial fractures do not cause operational failures, arcing often occurs across the fractures and results in further erosion of the solder of the solder joint. Such erosion exacerbates fractures in the solder joint and increases the likelihood of failures in the circuitry.
Accordingly, there is a need for techniques that eliminate, or at least significantly reduce, the forces being imparted on the solder joints within electronic modules that employ potting materials.